This invention relates to an improved system and method to promptly detect the vibration caused by contact between a cutting tool and workpiece without false alarming on machine tool noises.
A Machine Tool Monitor detects the sudden increase in vibration level when a slowly advancing tool insert first touches the workpiece. Prompt detection of the tool touch event before the tool advances far enough to mar the workpiece requires setting a low vibration threshold for the detection decision. On some machine tools spiky noise of short duration but high amplitude is encountered. If measures are not taken to prevent false alarms on these noise spikes, erroneous data on the location of the workpiece surface will be generated. The touch features of the Machine Tool Monitor can be used to set tool offsets and for on-line measurement of part dimensions.
Noise spikes encountered on production lathes in testing acoustic touch detection equipment have amplitudes as high as several hundred times the tool touch detection threshold that must be set to assure prompt detection of the tool touch event. However, noise spike duration has consistently fallen in a narrow band from 8 to 14 milliseconds, measured at the touch threshold level, indicating that the major factor determining spike duration is the anti-aliasing filter in the touch detection system analog preprocessing circuitry. The cutoff frequency of this filter is low enough to prevent aliasing from the subsequent sampling operation in the digital subsystem. The sampling period is long enough to accomplish digital analysis of the signal between analog signal samples. Therefore, it is not desirable to increase the filter cutoff frequency to shorten the noise spike duration.
This invention is an improvement over U.S. Pat. No. 4,428,055 to J. R. Zurbrick and J. R. Kelley and over copending application Ser. No. 645,203, filed August 29, 1984, now U.S. Pat. No. 4,631,683, Thomas et al, "Acoustic Detection of Contact Between Cutting Tool and Workpiece". The latter eliminates the false alarm problem by delaying the tool touch alarm for a period longer than the known maximum duration of such noise pulses; this is 15 milliseconds for the example above, which permits the tool to advance after the initial touch about 0.2 mil into the workpiece at a nominal touch test advance rate of one inch per minute. This delay, added to other unavoidable system delays, is only marginally acceptable since it requires increasing measurement time by use of slower tool advance rates if the system is to be used for other than initial and intermediate cuts, where some part marring is acceptable. It has been suggested, in an analog circuit approach, to test for slope polarity after crossing the detection amplitude threshold and waiting long enough to pass the noise pulse peak, and dismiss the signal if the slope is negative. There is a reducticn in delay time to alarm, but occasional false alarms occur due to multi-peaked noise pulses.